Automatic control of multi-speed vehicle transmissions is well-known. Speed ratio changes are effected automatically, usually by reference to vehicle speed and accelerator pedal position. Thus typically an upshift will be delayed when accelerator pedal position is indicative of a demand for rapid acceleration, allowing engine speed to be greater at the shift point than for the case of a demand for lesser acceleration. The timing of a downshift may be adjusted in a similar manner.
Electronic control techniques permit automatic speed ratio change to be initiated according to an electronic shift map whereby factors such as road speed, engine output torque, and accelerator pedal position are provided as inputs to an electronic processor, and algorithms and/or look-up tables allow the instant shift point to be determined.
Many control systems for automatic transmissions include gradient recognition, whereby the shift point may vary according to the instant road gradient. Thus when travelling uphill, a lower speed ratio may remain engaged due to the raising of a shift point with respect to engine speed. Conversely a shift point may be at a reduced engine speed when on a downhill gradient. In practice a plurality of shift maps corresponding to gradient steps may be provided. These arrangements may be used to improve fuel efficiency of a vehicle engine as compared with a shift map which is insensitive to gradient, and may also better meet driver expectations.
One method of gradient determination compares instant engine torque against acceleration (rate of change of speed), and provides an algorithm to calculate gradient by reference to deviation from base data obtained from running a vehicle on a level road. Other factors which may be taken into account include vehicle mass, altitude and ambient temperature.
If the vehicle accelerates faster than indicated by base data, it may be assumed that the vehicle is on a down gradient, and if slower is on an up gradient. Likewise a level gradient can be detected after an up or down gradient has ceased. This measure may also be used to compensate for changes in acceleration due to high vehicle mass or due to a towed trailer.
One characteristic of the known method of gradient determination is that it may be influenced by application of the vehicle brakes. In particular if the vehicle brakes are applied, the gradient determination algorithm (relying upon engine torque and acceleration) becomes inaccurate because part of the torque generated by the engine is lost in overcoming braking torque. In such circumstances, the shift points of the transmission may be inconsistent, and the vehicle driver may wrongly conclude that a malfunction is indicated.
An apparatus and method of overcoming this effect would be desirable.